One of the most promising technologies for lightweight, portable and eﬃcient electrical energy storage is the lithium ion (Li-ion) battery. State-of-the-art batteries comprise a cathode material containing lithium, a graphite anode that can store lithium ions between graphite sheets, and an electrolyte (often organic carbonates) that acts as the ‘blood’ of the battery.

In some cases, to increase the energy density of the battery, an alloy-type anode such as silicon, aluminium, germanium or tin, is used instead of graphite. However, graphite remains the most commonly used anode because of its good reversible capacity and cyclability.

Reactions at the interface

One of the most important elementary physical chemistry processes in a battery is the reaction at the interface between the electrode and the electrolyte. During the ﬁrst discharging process, graphite reacts with the electrolyte to form a passive ﬁlm on the electrode surface, referred to as solid electrolyte interphase (SEI).

This ‘sacriﬁcial’ layer plays a critical part in the electrochemical reactions of a graphite electrode. Its physicochemical properties determine the reversible storage of lithium ions in the graphite electrode (lithiation). SEI also strongly aﬀects lithium ion transport across the electrode−electrolyte interface and restricts further electrolyte decomposition to improve the cyclic performance of the electrode.

This means that SEI plays a vital role in the operation, safety and cyclability of Li-ion batteries [1]. For example, continuous growth of SEI is the main reason for long-term degradation of large-scale Li-ion batteries as available lithium ions are gradually lost.

Understanding SEI formation on anode materials, especially graphite and silicon, is important for developing high-performance Li-ion batteries. Researchers have used a variety of techniques to investigate how SEI forms in three dimensions, its composition, stability and inﬂuence on battery performance. But studying SEI is challenging as the layer only exists in situ, is chemically complex, fragile and keeps changing.

Image Credits: Morguefile/Alvimann

Electrochemical atomic force microscopy (EC-AFM)

However, now electrochemical atomic force microscopy (EC-AFM) is helping researchers probe deeper into SEI. AFM is a type of scanning probe microscopy with sub-Ångstrom resolution. It gathers data on the mechanical and electrical properties of materials and surfaces by ‘feeling’ the surface with a cantilevered mechanical probe controlled by piezoelectric components.

An AFM traditionally operates in either contact or tapping mode. In contact mode, the tip is ‘drawn’ across the sample in contact with the surface, which is mapped by recording the deflection of the cantilever directly [2]. In tapping mode, the tip oscillates, moving on and off the surface to give a high-resolution image [3].

More recently, PeakForce Tapping® has emerged as a third alternative [4]. Similar to tapping mode AFM, it was introduced by Bruker to increase the resolution into the sub-Ångstrom range using piconewton (pN) force control.

In PeakForce Tapping, the probe periodically taps the sample and the pN-level interaction force is measured directly and instantly by the deflection of the cantilever. Both tip and sample are better protected than in tapping mode.

Furthermore, the instant force detection at a single point in space (as opposed to the cycle averaged ‘amplitude’ used in tapping) allows straightforward operation at the sweet spot for resolution, both in air and in liquid. Finally, as a complete force curve is acquired for every image pixel, complete and unambiguous nano-mechanical information is automatically available.

These advantages make PeakForce Tapping particularly useful in imaging the extremely fragile SEI layer, which can be very challenging with conventional AFMs. Systems, such as Bruker’s Dimension FastScan AFM, equipped with PeakForce Tapping allows researchers to gather nanoscale data at a very high resolution.

In addition, PeakForce Tapping vastly simplifies high-resolution imaging in liquid on fragile samples and with high stability by eliminating the need for resonant cantilever tuning and enabling the use of automated ScanAsyst® imaging optimization.

Bruker's Dimension Icon® AFM platform, which can perform electrochemical studies when paired with the EC-AFM accessory.

How can PeakForce Tapping EC-AFM improve research into Li-ion batteries?

SEI undergoes substantial deformations when the underlying electrode particles expand and contract during cycling. This is particularly true for high-capacity anodes such as silicon.

Researchers at Brown University working with Bruker Nano Surfaces and General Motors Global R&D Center have used PeakForce Tapping EC-AFM in a new approach to study these deformations to gain new in-depth knowledge of how SEI fractures [1]. It improved on prior studies in several key ways, for example by providing substantially higher spatial resolution.

After applying controlled strains to SEI ﬁlms on patterned Si electrodes, the team used the technique to monitor how SEI degrades during cycling. In this way, they confirmed that cracks form when lithium ions become incorporated into the electrode (lithiation). This is the first time this has been observed directly.

Unexpectedly, they also found that additional SEI formation at low potentials did not ﬁll these cracks completely. Their experiments also made it possible to estimate the fracture toughness of the SEI, which was a key value that had not been previously measured.

PeakForce Tapping EC-AFM has also enabled researchers from the Chinese Academy of Science to investigate how the concentration of electrolyte affects the morphology on the interface with a graphite electrode [5]. The team reported direct evidence of signiﬁcant diﬀerences at the interface depending on the concentration of electrolytes.

For example, they found that with a concentrated electrolyte based on dimethyl sulfoxide (DMSO), stable ﬁlms form mainly at the step edges and on defects on the graphite surface after initial cycling. On the other hand, in the dilute electrolyte, they observed serious decomposition of the solvent and structural deterioration of the graphite surface, as DMSO-solvated lithium ions constantly pushed into the graphite layers.

Conclusion

As discussed above, PeakForce Tapping EC-AFM gives researchers an imaging tool with very high resolution and excellent force control in real time, allowing them for the first time to directly observe what’s happening in the battery as electrochemical reactions occur.

These capabilities are helping researchers measure properties more precisely at the fragile interface of electrode and electrolyte, which should lead to even better designs for the production of high-performance Li-ion batteries.

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